Contemporary telecommunication systems are increasingly digital in nature. That is, switching systems convert received analog information into digital data to perform their function and/or are adapted to receive digital data from various sources. It is increasingly common to connect peripheral equipment such as integrated voice-data sets to such systems using a data link carrying a time-multiplexed digital data stream. The data stream is usually channelized with each channel containing a plurality of bits, some of which are used for digitized pulse code modulated (PCM) speech or user digital data and others of which are used for signalling between the terminal and the switching sytems.
One common serial channelized data stream is known as DS-30X formatted data. It uses a 2.56 Mb/s data link whose format comprises a frame having 32 channels each of ten bits. Eight of the bits are used for PCM speech or user digital data, one bit is variously used for special control signals and one bit is used as a signalling bit. With a frame period of 125 microseconds the eight bits provide a clear 64 kb/s channel between the user terminal and the switching system whereas the one signalling bit provides an 8 kb/s signalling channel.
The signalling channel created by the signalling bit from each channel is only needed occasionally, as for example when a user dials a digit at the particular terminal associated with that channel. When the signalling channel is not transmitting intelligence, the binary level of the pulse in that bit time slot is repeatedly maintained at one level, typically a binary 1 (ONE) or so-called marking voltage level, for all the frames during the idle interval. When a terminal has a signalling message to send, it transmits a binary pattern which is not all binary 1's to indicate the beginning of a message. It is also desirable that the terminal informs the receiving circuit of the length of the message to expect. The header of a signalling message may therefore comprise a start message, for example a zero bit followed by three length bits which express the length of the message as a value from zero to seven. Of course, any combination of bits may be used as the signalling message header.
It is therefore a requirement for the data receiving equipment at the switching office to be able to partition the received data stream into the separate channels and strip out the signalling bits from the channels so that the signalling message may be recognized and acted upon.
An object of this invention is to provide a system for receiving variable length signalling messages embedded in a serial channelized data stream wherein the message extraction circuitry comprises a storage device which needs to be only large enough to store one byte of signalling message.
It is another object of this invention to provide a single message extraction and data handler circuit for extracting signalling data from a plurality of data streams received simultaneously.